This invention relates generally to moisture measuring apparatus and, more particularly, to capacitive moisture measuring apparatus for measuring the moisture content of granular material.
Moisture measuring apparatus which utilize capacitors whose capacitance vary as a function of the moisture of the material being measured are known. For example, in the measurement of the moisture of granular material, and, in particular, in measuring the moisture of grain, it is known that the moisture content of the material usually varies in the range of 10 to 40%. However, it is particularly important to obtain the highest possible degree of accuracy in moisture measurement in the moisture range of from about 12 to 17%. The use of a capacitive measuring apparatus when the moisture level falls within this range has proven to be especially advantageous since the dielectric constant of such material varies in a substantially linear manner within a moisture range of from about 4 to 18%. Since the dielectric constant varies in an abrupt manner as the moisture content exceeds 18%, measuring apparatus utilizing variations in resistance obtain greater accuracy than capicitance methods and apparatus in that range.
Moisture measuring apparatus utilizing the moisture dependent variations in the capacitance of a capacitor are disclosed in U.S. Pat. Nos. 3,508,435 and 3,626,286.More particularly, the measuring apparatus disclosed in U.S. Pat. No. 3,508,435 utilizes a series circuit comprising a resonance circuit and a resistor, the series circuit being driven by an oscillator supplying a voltage at a frequency of 15 kHz. The moisture measuring apparatus disclosed in U.S. Pat. No. 3,626,286, utilizes an oscillator which supplies a peak to peak voltage of 100 volts at a frequency of 75 kHz to a measuring circuit comprising two capacitors, one of the capacitors functioning as a measuring capacitor while the other capacitor having a known capacitance. In this latter apparatus the detector input voltage is derived from the voltage division effected by the pair of capacitors.
Conventional capacitive moisture measuring apparatus of the type described above have proven to be not entirely satisfactory. Thus, such prior art apparatus are extremely complex in construction and, accordingly expensive in manufacture. Accordingly, such prior art capacitive moisture measuring apparatus are not particularly suited for manufacture in mass production. Further, by virtue of their relatively complicated design, such prior art measuring apparatus are not as reliable as is desired.